End-pumped lasers offer many advantages over side-pumped lasers in efficiency and beam quality. This is true if all of the pump energy can be imaged inside the fundamental mode radius of the solid-state laser, and if the gain medium is long enough to absorb a large fraction of the pump light.
The evolution of diode lasers to higher output powers has been from an array of single strip diodes or extended emitter diode lasers, to a bar of multiple arrays, and then to a stack of bars. Examples of diode laser arrays are Spectra Diode Labs Models SDL-2460 and SDL-2480, or Sony Models SLD 301, 302, 303 and 304. The emission pattern separation of these arrays is less than a few microns in the direction perpendicular to the diode junction. In the direction parallel to the diode junction, the pattern length is almost one millimeter for the higher power diode arrays. Because of thermal load requirements, the next step to higher power is presently implemented by use of a diode laser bar of many arrays. The Spectra Diode Labs Models SDL-3490 and 3450 are examples of 10-Watt and 15-Watt laser diode bars. These bars still emit from a few micron source perpendicular to the plane of the junction, but for the other plane the length is typically 10 mm. The number of arrays in these bars is typically between 10 and 35.
For even higher laser output power, the diode bars can be stacked into two-dimensional arrays. The model SDL-3230-TZS units are bars stacked on 0.4 mm center-to-center spacing. These stacked bars are rated only for quasi-cw operation due to thermal constraints, but they are rated for 60 Watts of power per bar at a few percent duty cycle.
T. Y. Fan et al, in "Scalable, End-Pumped, Diode-Laser-Pumped Laser," Optics Letters, Vol. 14 (1989) pp. 1057-1059, and in "Pump Source Requirements for End-pumped Lasers", I.E.E.E. Jour. Quant. Electr., vol. 26 (1990) pp. 311-316, describe a technique for scaling of end-pumped lasers using diode laser arrays. The light beam from each diode laser is collimated in two perpendicular directions by two consecutive cylindrical lenses to produce a light beam with parallel edges. This light beam is then focused by a lens on an end of an optical fiber with a large numerical aperture (NA.gtoreq.0.47) to pump a laser gain medium.
Anderson et al disclose use of a cylindrical lens to collimate light issuing from a diode laser to produce a light beam with parallel beam edges, in U.S. Pat. No. 3,736,518.
Barry et al, in U.S. Pat. No. 4,383,318, disclose capture of light issuing from a plurality of diode lasers by a plurality of optical fibers positioned at faces of the diode lasers. These optical fibers are then caused to coalesce into a single optical fiber, which delivers the diode laser light captured therein for side pumping of a laser gain medium. No optics are disclosed for capture or control of the light beams emitted by the diode lasers.
U.S. Pat. No. 4,710,940, issued to Sipes, discloses use of an optical cavity with multiple light beam bounce points along two transparent parallel edges of the cavity. Individual laser diode arrays are positioned to direct diode laser light beams at the bounce points. No optics are disclosed for capture or control of the light beams emitted by the diode lasers.
Baer, in U.S. Pat. No. 4,785,459, discloses use of a zig-zag cavity resonator with a diode laser pump positioned at each bounce point of the zig-zag optical path in the resonator. Each diode laser is either butted against a side of the resonator or is passed transversely through an optical fiber (shown most clearly in FIGS. 3A-3E of the Baer patent) to collimate the diode laser light and produce a light beam with parallel beam edges in a first direction. The resulting light beam is delivered to a laser gain medium within the resonator.
What is needed is an approach that collects and concentrates laser output from a stack of diode laser bars in a form that is useful and flexible for pumping of a laser. Preferably, the approach should take account of the difference in beam divergence angles for light emitted by a diode laser in two perpendicular directions.